1. Field of the Invention
The present invention relates to fluid mass flow controllers, and in particular to controllers for maintaining a constant rate of flow of a fluid despite fluctuations in the fluid supply pressure.
2. The Prior Art
There are many processes that require precise control of the rate of flow of a fluid. For example, during fabrication of semiconductor wafers in a furnace, the motion and composition of the atmosphere within the furnace must be precisely controlled. Such control can be achieved by using a plurality of fluid mass flow controllers, one for each gas that is required for the particular process then being carried out. Each controller maintains the flow of its associated gas into the furnace at a predetermined rate, thereby maintaining the desired partial pressure of that gas in the furnace and preserving a smooth, steady motion of the atmosphere through the furnace.
In a typical fluid mass flow controller, the rate of flow of a gas is sensed and is compared with the desired rate. If the rate of flow as measured by the sensor does not coincide with the desired rate, a valve located down stream from the sensor is opened or closed as necessary to cause the actual rate of flow to equal the desired rate.
In a modern semiconductor manufacturing plant, several different furnaces may be operating at any one time and a different process may be taking place within each furnace, all under computer control. As a process being performed in a given furnace comes to an end, the computer may direct the flow controllers regulating the gases flowing into that furnace to shut off, or it may direct them to change the various rates of flow in preparation for another process. Each occurrence of a flow controller shutting off or changing a flow rate will cause the pressure throughout that controller's associated gas supply line to fluctuate, and this pressure fluctuation will be observable at the inlets to the other flow controllers connected to the same gas supply line.
Although existing flow controllers can accurately maintain a required rate of flow of a gas and can respond quickly to a computer command to change to a new rate of flow, they do not respond well to fluctuations in inlet pressure, especially at relatively low flow rates. A relatively small fluctuation in inlet pressure can result in a large fluctuation in the gas flow rate if the rate being maintained is relatively low. Such a fluctuation in gas flow can stir up particulate matter, ruining semiconductor wafers then being fabricated. In addition, if the affected gas is one that must be present in a precise quantity, such a large fluctuation can disrupt the fabrication process entirely. It will be apparent that better quality control and a higher yield could be achieved in the semiconductor manufacturing process if such fluctuations in gas flow rate could be prevented.
If the volume of gas between the sensor and the valve were to be made smaller, the effect of inlet pressure fluctuations on the gas flow rate would be reduced, and therefore it has been proposed to solve the problem by modifying the structure of the controller so as to greatly reduce this volume. However, the sensor cannot occupy the same space as the valve and hence it is not possible to eliminate this volume of gas entirely. Moreover, there are practical limits on how small the enclosing conduits can be made before susceptibility to clogging becomes significant. Accordingly, there are finite limits to how small the volume of gas between the sensor and the valve can be, and this proposed solution to the problem is not practical.
It will be apparent from the foregoing that there is a need for a fluid mass flow controller that can maintain a constant rate of flow of a fluid despite fluctuations in the pressure of the fluid at the inlet to the flow controller. The present invention satisfies this need.